1. Field of the Invention
The present invention relates to a semiconductor device having an IGBT structure and to a method for manufacturing the semiconductor device.
2. Description of the Related Art
An insulated gate bipolar transistor (IGBT) has a high input impedance and a low ON voltage, and accordingly, is used for a motor drive circuit and the like. However, in the IGBT, the withstand voltage and the ON voltage have a trade-off relationship.
Therefore, a variety of methods are proposed in order to reduce the ON voltage while maintaining a high withstand voltage. For example, a method of forming a carrier accumulation layer between a base region and a drift region is proposed. It is disclosed that, in accordance with this structure, carriers are accumulated in the vicinity of the interface between the carrier accumulation layer and the drift region, and the ON voltage can be reduced while maintaining a relatively high withstand voltage.
However, though the ON voltage is suitably reduced, the withstand voltage is also lowered since it becomes difficult to extend the depletion layer of the carrier accumulation layer due to the fact that the impurity concentration of the carrier accumulation layer is raised. As described above, it is difficult to enhance both the withstand voltage and the ON voltage. Moreover, in a semiconductor device with such an IGBT structure, a latch-up phenomenon occurs by generation of parasitic transistors.